This application is based on, and claims priority to, Japanese Application No. 2002-062095, filed Mar. 7, 2002, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical modulator and a design method therefor. More particularly, the present invention relates to an optical modulator that utilizes an electro-optic effect of a crystal substrate. The invention also relates to a method of designing an optical modulator which performs electrical-to-optical conversion by modulating a given light beam with an electrical signal.
2. Description of the Related Art
Recent years have seen an increasing use of multimedia applications, with a growing awareness of demands for more advanced optical communications networks that provide higher speeds and larger bandwidths. Optical modulators are one of the key devices for realizing such high-performance optical networks. One type of optical modulator is an external modulator, which performs electrical-to-optical conversion by modulating an incoming light beam with an electrical signal. The modulating signal produces an electric field across an optical waveguide fabricated on a substrate, so that the light beam propagating along the waveguide will be varied in phase as a result of interaction between the light and the electric field being applied to it.
To meet the recent demand for high-speed, high-bandwidth optical communication, technological migration from 10 Gbps-class systems to 40 Gbps-class systems has begun, including the deployment of dense wavelength-division multiplexed (DWDM) optical transmission systems. The new systems require optical modulators to operate four times faster than before. To fulfill this requirement, it is necessary to reduce the drive voltage of modulators since high-speed electronic circuits cannot produce a large voltage swing.
In designing such an external optical modulator as mentioned above, however, we trade off faster operating rates (or wider modulation bandwidths) against lower drive voltages. In general, we can increase the modulation rates if the electric capacitance is small. This would be accomplished by simply cutting the length of the optical waveguide (or actually, reducing the length of a signal electrode that makes a modulating electric field interact with the light beam traveling on the optical waveguide). The reduction of this xe2x80x9cinteraction length,xe2x80x9d however, also reduces the amount of resulting phase displacements, causing a decreased modulation ratio. Contrary to our desire for a lower drive voltage, we now have to increase the drive voltage to yield a sufficient modulation depth. For this reason, there have been difficulties in further improving the performance of conventional optical modulators or reducing the drive voltage for optical modulators.
In view of the foregoing, it is an object of the present invention to provide a high-speed, high-performance optical modulator which operates with a reduced drive voltage without sacrificing its modulation bandwidth.
It is another object of the present invention to provide a method to design a high-speed, high-performance optical modulator which operates with a reduced drive voltage without sacrificing its modulation bandwidth.
To accomplish the objects stated above, according to the present invention, there is provided an optical modulator including an optical waveguide fabricated on a crystal substrate that exhibits an electro-optic effect; a signal electrode placed in the vicinity of the optical waveguide; and ground electrodes formed on both sides of the signal electrode. The characteristic impedance of the signal electrode is set within a range where microwave reflection is limited below a predetermined level. The light beam traveling along the optical waveguide is phase-matched with a microwave signal traveling along the signal electrode. The gap between the signal electrode and ground electrodes is at least 44 xcexcm, while the interaction length of the signal electrode is at least 41 mm. With such a setup, the light beam can be modulated at a rate of 40 Gbps or higher.
Objects of the present invention are achieved by providing an optical modulator including (a) a ridge; (b) a signal electrode on the ridge, the signal electrode having an interaction length which is at least 41 mm; and (c) a ground electrode. A gap width between the ground electrode and the signal electrode is at least 44 xcexcm.
Objects of the present invention are also achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) a ground electrode; and (e) a signal electrode on the ridge. A gap width between the ground electrode and the signal electrode is at least 44 xcexcm. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
Moreover, objects of the present invention are achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and positioned between the first and second ground electrodes. A gap width between the first ground electrode and the signal electrode, and between the second ground electrode and the signal electrode, is at least 44 xcexcm. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
Objects of the present invention are also achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a z-cut LiNbO3 substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and positioned between the first and second ground electrodes. A gap width between the first ground electrode and the signal electrode, and between the second ground electrode and the signal electrode, is at least 44 xcexcm. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A buffer layer is between the signal electrode and the ridge. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
In addition, objects of the present invention are achieved by providing an optical modulator including (a) a crystal substrate that exhibits an electro-optic effect; (b) an optical waveguide through which a light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and between the first and second ground electrodes. A gap width between the signal electrode and the first ground electrode, and between the signal electrode and the second ground electrode, is at least 44 xcexcm. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. The light traveling through the optical waveguide is phase-matched with a microwave signal traveling through the signal electrode. Characteristic impedance of the signal electrode is set within a range where microwave reflection is limited below a predetermined level.
Objects of the present invention are further achieved by providing an optical modulator including (a) a z-cut LiNbO3 substrate; (b) an optical waveguide through which a light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and between the first and second ground electrodes. A gap width between the signal electrode and the first ground electrode, and between the signal electrode and the second ground electrode, is at least 44 xcexcm. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A buffer layer is between the signal electrode and the ridge. The light traveling through the optical waveguide is phase-matched with a microwave signal traveling through the signal electrode. Characteristic impedance of the signal electrode is set within a range where microwave reflection is limited below a predetermined level.
Objects of the present invention are achieved by providing a 40 Gbps optical modulator including (a) a substrate; (b) a signal electrode formed on the substrate, the signal electrode having a base with a width W; and (c) a ground electrode. A gap width S exists between the ground electrode and the signal electrode. The ratio SAN is greater than or equal to 8.
Objects of the present invention are further achieved by providing a 40 Gbps optical modulator including (a) a substrate; (b) first and second ground electrodes formed on the substrate; and (c) a signal electrode formed on the substrate between the first and second ground electrodes. A gap width S exists between the first ground electrode and the signal electrode, and between the second ground electrode and the signal electrode. The signal electrode has a base with a width W. The ratio S/W is greater than or equal to 8.
Objects of the present invention are also achieved by providing a method of designing an optical modulator which performs electrical-to-optical conversion by modulating a light beam with a microwave signal. The method includes (a) defining an allowable range of characteristic impedance within which microwave reflection is limited below a predetermined level; (b) performing phase matching by making effective refraction index for the microwave signal agree with that for the light beam; (c) defining a first relationship that associates the thickness of ground electrodes with the width of a gap between a signal electrode and the ground electrodes, based on a result of said phase matching step (b); (d) defining a second relationship that associates ground electrode thickness with the gap width within the allowable range; (e) determining an acceptable range of the gap width and the ground electrode thickness, based on the first and second relationships; (f) plotting the driving voltage and an interaction length of the signal electrode within the acceptable range; and (g) obtaining optimal values of the gap width, interaction length, drive voltage, and ground electrode thickness by increasing the gap width together with the interaction length to reduce the drive voltage and loss of high-frequency components of the microwave signal.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.